366 
GREENLAND VOYAGE. 
strata a, b, be of unequal temperature, (say a — 40 J ; 
b — 50°) andjnoving eitlier with different veloci¬ 
ties, or in different directions. Their unequal 
motion would produce an admixture, which, in 
the case of their being perfectly humid, must oc¬ 
casion condensation. The vapour thus condensed, 
would descend by the action of gravity, and would 
continue to present the appearance of a cloud, so 
long as it was passing through absolutely damp 
air of uniform temperature; but on falling into a 
third stratum, c, of dryer air, of a similar tempera- 
tine, it would be absorbed and disappear. This 
would mark the lower edge of the cloud. But 
should the descending vapour, instead of a third 
stratum of less humid air, meet with a continued 
stratum, damp to saturation, it must fall to the 
earth; and if the temperature of the lower air 
should be higher than that of the vapour, the air 
would lose some of its heat in raising the tempera¬ 
ture of the vapour to a mean, which would cause 
a deposition of moisture from this stratum also, 
(because, if it were perfectly humid in its former 
temperature, it could not contain all its moisture 
under a less heat), whereby the aqueous parti¬ 
cles, being now brought nearer together by their 
increased numbers, would have a greater tenden¬ 
cy to coalesce, until they were enlarged to the 
magnitude of drops, that should be productive of 
